The combined model and enzyme studies proposed in this project are directed toward developing a detailed electronic structure description pyranopterin molybdenum active sites, and how their unique electronic structures relate to their mechanism of activity. Computational studies will provide a valence bond description of these active sites, and aid in our understanding of enzyme-substrate interactions and the nature of the transition state. The specific aims of the project are toe 1) provide insight into the nature of the xanthine oxidase (XO) 'Very Rapid' intermediate, and assess the proposal that this intermediate is an product bound species, 2) determine the effect of terminal sulfido protonation on Mo-S bond covalency, and how this might facilitate electron transfer regeneration of the XO active site, 3) incorporate selenide into the XO site and perform spectroscopic studies on the chemically modified form of the enzyme, 4) use spectroscopically calibrated bonding calculations to probe the reaction coordinate for XO mediated hydroxylation of aldehyde and heterocyclic substrates, 5) determine whether the unique geometry of the oxidized sulfite oxidase (SO) active site directs a specific oxo ligand for transfer to substrate in the reductive half reaction, effectively lowering the energy of the transition state, 6) ascertain the role of the coordinated cysteine and ene-1,2-dithiolate donors in the oxidative half reaction of SO, 7) understand how the A208D mutation in human SO affects Mo-S-Cys bonding in SOox and/or SOred, 8) determine the role of the ene-l,2-dithiolate chelates in the electron transfer (reductive) half reaction of DMSO reductase (DMSOR), and 9) understand the role of the coordinated serine in catalysis, and probe the nature of the DMSOR transition state. The enzymes XO, aldehyde oxidase (AO), and SO are found in humans, and their importance with respect to human health is exemplified by the fact that individuals suffering from molybdenum cofactor deficiency display severe neurological symptoms and early childhood death. Enzymes of the XO family have recently been implicated in pro-drug activation, drug metabolism, and under specific conditions NO synthase activity, and AO has recently been shown to metabolize famciclovir to the potent antiviral penciclovir, which has been found to be effective against such viral infections as herpes simplex, varicella zoster, Epstein-Barr, and hepatitis B. Individuals who suffer from isolated sulfite oxidase deficiency, which derives from specific mutations in the SO gene, display a variety of deliterious effects including neurological abnormalities, dislocation of the ocular lens, mental retardation, and even attenuated brain growth.